Non-rebreathing valves or NRVs are commonly used in an assortment of anesthesia administration equipment and respiration assistance apparatus including, inter alia, ventilators, resuscitators and sleep apnea treatment devices. The NRV is typically situated in the breathing apparatus gas flow circuit between a source of respiratory gas (e.g., ambient or pressurized air, pressurized oxygen and/or anesthetic gas) and a patient interface means such as a nasal or oral/nasal mask, an endotracheal (intubation) tube or nasal prongs. The function of the NRV is to act essentially as a two-way check valve. More particularly, when it is desired to deliver respiratory gas to the patient, the NRV permits such flow. When the patient exhales, however, the NRV vents the patient's expiratory gases while temporarily stopping the flow of respiratory gas responsive to back pressure created by the patient's expiratory efforts. In this manner, the NRV effectively prevents mixing of the patient's expiratory gases with the delivered respiratory gas whereby the patient does not "rebreathe" his expiratory gases.
Although their functions are essentially the same, NRVs assume a broad variety of structural configurations and levels of functional sophistication. Because of its relative simplicity in construction and low resistance to administered respiratory gas flow, a commercially popular NRV is the type commonly known as a "duck-bill" valve. A duck-bill valve derives its name from the peculiar shape of its valve element. That is to say, a duck-bill valve element typically comprises a thin, resilient diaphragm that is secured at its periphery to a valve housing and from which projects, in the direction of administered respiratory gas flow, a hollow, wedge-like extension that terminates in a small slot and generally resembles the shape of a duck bill.
The duck-bill valve element is constructed such that its slot is normally closed. However, in response to a flow of respiratory gas, which may arise from negative pressure associated with a patent's inspiration and/or delivery of respiratory gas under positive pressure, the slot opens to permit the respiratory gas to flow to the patient's airway. When the patient thereafter exhales, the back pressure exerted by the patient's expiratory gases closes the slot and displaces the valve element from its valve seat whereupon the expiratory gases are diverted to and discharged from suitable exhaust port means provided in the valve housing.
Moreover, duck-bill NRVs, like many other NRVs, may be employed simply to exhaust the patient's expiratory gases or, alternatively, they may be used in conjunction with mechanisms that are designed to create a phenomenon known as positive end expiratory pressure or PEEP. PEEP is desirable in certain instances, e.g., in assisting the breathing of a chronic pulmonary obstruction disorder (COPD) patient, where it is necessary to effect a somewhat elevated resistance to the patient's expiratory efforts to thereby promote the onset of inspiration at the end of the expiration phase of the patient's respiratory cycle. Typically, the mechanisms for effecting PEEP are adjustable to set the appropriate level of respiratory resistance.
Examples of presently known duck-bill valves are provided in U.S. Pat. Nos. 3,363,833, 3,556,122, 4,774,941, 5,109,840 and 5,279,289. Ironically, the primary feature which renders duck-bill valves particularly desirable for use in breathing apparatus, namely, a thin, flexible valve element that offers minimal resistance to respiratory gas flow, is a source of potentially serious malfunctions in such valves. Specifically, should the exhalation efforts of the patient be extremely forceful, such as, for example, when the patient coughs, the sudden imposition of high-level impulses of back pressure on the valve element may cause the duck-bill portion of the valve element diaphragm to invert. Under these circumstances, the duck-bill would point in the direction of the administered respiratory gas flow and the slot thereof would be caused to close under the influence of the applied respiratory gas. As a consequence, the supply of respiratory gas to the patient would become effectively occluded whereby the patient may experience harmful or even fatal respiratory distress, particularly if the patient is unconscious or is not being closely monitored by medical personnel.
Perhaps recognizing although not specifically identifying the need to prevent inversion of the duck-bill portions of their valve elements, the valves disclosed in U.S. Pat. Nos. 3,363,833, 3,556,122 and 5,109,840 disclose valve housings which incorporate various and sometimes elaborate structures upstream of the duck bill which permit respiratory gas to flow through the duck bill but, by virtue of their location, would appear to prevent the duck-bill from inverting. U.S. Pat. No. 5,279,289, on the other hand, expressly provides for a retainer ring upstream of the duck-bill valve element to "support" the valve element. In any event, even if the valve housing components disclosed in these patents effectively prevent inversion of the duck-bill, the very presence of such structures renders the valves unduly complicated in design and, therefore, commensurately expensive to manufacture.
An advantage exists, therefore, for a duck-bill non-rebreathing valve which is simple in design, economical to manufacture and which precludes inversion of the duck-bill valve element under extreme operating conditions.